This invention relates generally to laser marking of products and, more particularly, relates to detection and indication of the distance between a laser marking apparatus and a product to be marked.
There exist many different ways to place printed matter on a product package, including ink printing on the product, laser marking on the product, and ink or laser preprinting of a label to be placed upon the product. Of these, laser marking the product is often the most desirable alternative because of its low cost and easy adaptation to different marking jobs. It is especially useful in putting final markings, such as expiration dates and the like, on completed products. The technique of laser marking entails directing a focussed beam of light generated by a laser at a product. The focussed laser light marks the surface by burning caused by absorption of the light and a transformation of that light, or photon, energy into heat energy. Alternatively, the photons of the focussed laser light may interact directly with the material of the target without an intermediate transformation into heat energy.
In either case, the degree of visual change in the target material will be a function to some extent of the energy density in the focussed laser light, which is partly a function of the beam focus. Additionally, the sharpness of the mark may also be affected by the degree of focus of the laser beam. Accordingly, it is important that the distance between the product to be marked and the laser light focussing lens closely match the focal length of the lens. Additionally, the laser light focussing lens is often changed in order to facilitate the making of different sized markings. It is desirable in such cases that an operator be able to easily and quickly adjust the distance between lens and product to match the desired distance.
Many lasers used for marking applications emit light that falls outside of the human visible spectrum, and thus visual optimization of the focus is impractical. Even where the laser light is visible, it is often difficult to visually optimize the focus. Accordingly, in the past, operators of industrial laser marking systems have manually measured the distance between the lens and product, using a ruler, and have adjusted the distance to match the known focal length of the lens. Alternatively, or in addition, the distance is sometimes varied as several trial pieces of product are marked, and the distance is fixed when the product appears to be properly marked. Neither of these methods is fast or easy, and the second method involves wasting the improperly marked trial pieces.
A method and apparatus are needed whereby a laser marking equipment operator may receive a human-perceivable indication of the distance from lens to product to be marked, so that such distance may be set equal to the focal length of the lens, or to another desired distance.
According to an embodiment of the present invention, a distance measuring sensor is mounted proximal to and at the same vertical height as a laser light focussing lens, in an industrial laser marking apparatus. Indicators actuated by the distance measuring sensor give a human-perceivable indication of the distance between the sensor and a target. Due to the proximal placement of the distance measuring sensor to the laser light focussing lens, the human-perceivable indication serves to indicate the distance between the laser light focussing lens and the target as well. The human-perceivable indication may be comprised of light emitting diodes (LED""s) which are activated selectively to indicate the measured distance.